A scale is formed normally on the surface of a wire material manufactured by hot rolling, and it is required to remove the scale before subjecting the wire material to secondary work such as drawing and the like. As such a scale removing method before secondary work, a batch type acid cleaning method was employed in prior arts, however, in recent years, from the viewpoints of the environmental pollution and cost reduction, a mechanical descaling (hereinafter referred to as MD) method has come to be employed. Therefore, the wire material is required to be formed with a scale with excellent MD performance.
As methods for manufacturing a wire material formed with a scale with excellent MD performance, Patent Literatures 1-4 can be cited for example. In Patent Literatures 1, 2, the scale amount remaining in the wire material after MD is reduced by forming a scale high in FeO ratio (or low in Fe3O4 ratio) and thick. In Patent Literature 3, by lowering the boundary face roughness, propagation of the crack occurring on the boundary face of the scale is promoted, and the remaining scale amount is reduced. In Patent Literature 4, by making the holes of 1 μm or more and 3 μm or less present by a constant amount in the scale, the scale adhesiveness is increased, and the peeling performance is improved.
However, Patent Literatures 1-4 described above have problems as described below. According to the method of forming the scale thick as Patent Literatures 1, 2, even when a bending strain is applied to the wire material by the MD method and the wire material surface is subjected to brushing, it is difficult to perfectly remove the scale. More specifically, according to the MD method, different from the batch type acid cleaning method, it is difficult to remove the entire scale evenly and stably, and even when the wire material formed with thick scale is subjected to MD, the surface of the wire material may occasionally be spotted with finely crushed scale powder. When the remaining scale remaining locally thus increases, in the secondary work such as drawing and the like, problems such as occurrence of a flaw due to the defective lubrication, lowering of the lifetime of the dice and the like are caused.
Also, it is difficult to stably lower the boundary face roughness by the method of lowering the boundary face roughness such as Patent Literature 3, it is difficult to stably form the holes even by the method of forming large holes of 1 μm or more inside the scale such as Patent Literature 4, and it is difficult to stably reduce the remaining scale amount according to either of these technologies.